The 3GPP (3rd Generation Partnership Project) specified IMS (IP Multi-media Subsystem) network is an access-independent and standard-based IP connectivity and service control architecture that enables various types of multimedia services to end-users using common Internet-based protocols.
As outlined in 3GPP TS (Technical Specification by the 3rd Generation Partnership Project) 23.292, T-ADS (Terminating-Access Domain Selection) shall be aware of currently used access (PS, Packet Switched, or CS, Circuit Switched) to forward terminating calls to a user. T-ADS shall also check for IMS voice over PS support and RAT (Radio Access Type) type in the serving MME (Mobile Management Entity) and/or SGSN (Serving GPRS Support Node). T-ADS shall furthermore, for all terminating calls for registered contacts (if registered via PS), query the current serving nodes (via HSS, Home Subscriber Server) for IMS voice over PS support and RAT type.
T-ADS obtains the aforementioned information (PS support and RAT) via a reference point Sh from the HSS. HSS obtains the information via reference points S6a to an EPC (Evolved Packet Core) node MME or via Gr/S6d to SGSN.
Service Domain Selection (SDS) is a concept used to decide on the serving domain for an originating call, for instance CS or IMS, as described in 3GPP TS 23.221 as well as when a call arrives at terminating GMSC (Gateway Mobile Switching Center) whether CS or IMS network should serve the call as described in 3GPP TS 23.221.
3GPP TS 23.292 describes a solution for a static domain selection, i.e., originated and terminated calls are always routed to the IMS.
SDS and T-ADS may also be considered in combination.
With introduction of IMS Centralized Services (ICS), user's services are migrated from the CS network to IMS. IMS will be the network serving the user as a single service engine meaning that originating and terminating calls need to visit IMS.
It is assumed that ICS is introduced aligned with start of VoLTE (Voice over LTE, i.e. Long Term Evolution) where MMTeI/IMS is the recommended service engine.